In certain driving situations a great tractional resistance can result, for example, when driving uphill with trailers, when repeatedly starting with only brief pauses, or when tow starting another vehicle in the off-road use. Automated starting clutches can become thermally overloaded and wear out more quickly in case of very great tractional resistance, but also when creep driving with stepped speed and conscious operating error, for example, due to “stall speed”, to a slip between parts and a heating resulting from an increased charge of energy. The consequence is damage to clutch parts, such as lined and steel discs, the same as the thermal decomposition of the transmission oil.
It should, therefore, be prevented that the thermal load limits of the clutch or of the oil be exceeded. Different methods have already become known which make the driver aware of an increased clutch load. For this purpose, optical or acoustical signals the same as clutch pulses are used, for example, which are emitted upon exceeding a limit value defined by a critical driving state starting from which an increased wear is to be feared. For defining such a limit value, different operating parameters are considered such as the friction work, the engine rotational speed, the temperature increase, or the slip value.
On the basis of such limit values, it is also possible to reduce the clutch load by the effect of the clutch control or of the engine control and thus to reduce the danger of a thermal overload. This can be done, for example, by a ramp-like increase of the clutch torque or in the engine by an injection scattering.
DE 198 15 259 A1 has disclosed a device for control of an automated clutch in the output train which device determines the clutch slip in the area of the clutch friction lining and with an actuation unit adjusts the torque transmitted by the clutch. By means of the clutch slip and of the transmitted torque are determined the energy charge in the area of the clutch friction linings, a temperature increase of the friction linings compared to a limit value, or an increased wear of the linings. Depending on an increased energy charge, a temperature increase or an increased wear compared to a limit value, the control unit controls in a first time phase the transmitted clutch torque not affected by the energy charge. In a second time phase, the torque transmitted by the clutch is controlled by time and in a third time phase the transmitted torque is increased until the engine rotational speed exceeds a threshold value or until a certain period of time has elapsed. At the same time in the second time phase a soft warning of the driver is effected by a pulsing motion and in the third time phase a clearly traceable shake or jerk is felt.
DE 196 02 006 A1 outlines a device and a method for control of a clutch in which an actuator controls the adjustment of the torque transmitted by the clutch. If the duration of time of an operating state with increased wear or increased thermal load is exceeded, this is indicated to the driver by a dynamic behavior of the drive train. At the same time, the vehicle remains movable in every situation with said dynamic behavior. Such a dynamic behavior results, for example, by control variable in time of the transmitted torque, specifically in a manner such that periodic, aperiodic or static fluctuations of the transmitted torque result. For the control are determined the friction charge in the friction faces of the clutch as function of time and the temperature of the clutch as function of time. System variables can be used for this purpose such as operating state, gear position torque, slip, engine rotational speed, transmission input rotational speed, suction pressure, speed signals, clutch adjustment signal, transmission output rotational speed, transmitted clutch torque, engine torque, temperature sensor signal, gradient sensor signal and gear position.
Most of the system variables used for control of the clutches are determined by simple measuring devices such as sensors and detectors or are determined by adjustment of the corresponding actuating device.
However, measuring of the temperature on the clutch is complicated. Measuring of temperature, via the outflowing coolant, is particularly expensive in clutches in which both halves are rotatable. It is also difficult to accommodate a temperature sensor close enough to the rotating parts.
DE 196 41 074 A1 has made known for determining the temperature of a starting clutch a method in which, based on the temperature of the environment of the starting clutch, the actual temperature thereof is calculated by integration of the temperature change. The differential temperature is here proportional to the difference from the friction work of the clutch and the heat loss, which difference is determined from the dissipated flow of the coolant.